The Apple is a food source that restores two hunger bar portions.
There are two ways in obtaining an apple in single player along with the rarer way of obtaining one in multiplayer.
The first method in single player is to find them in chests in dungeons, abandoned mineshafts, strongholds, and NPC villages.
The second way is breaking leaves on trees. There is roughly a 1 in 200 chance of getting an apple from a leaf block. The rarer method to getting an apple on multiplayer is to find and kill Notch. When Notch is killed, he not only drops his normal inventory, but an apple as well.
Related ThreadsGolden Apple In First Dungeon!? - last post by @ Mar 19, 2012
All Apples No stick!-Minecraft hunger Games - last post by @ May 11, 2014